Putting a stent indwelling in a blood vessel has been practiced for treating a heart disease or the like. In general, the material constituting a stent is a metallic material such as stainless steel, a cobalt-chromium alloy and a nickel-titanium alloy. After the stent is put indwelling in a blood vessel, a series of healing process such as growth of smooth muscle cells and formation of an endoththeliocyte layer proceeds following to a thrombogenic reaction and an inflammation reaction. In recent years, drug-eluting stents for the purpose of preventing excessive growth of smooth muscle cells or intravascular restenosis arising from the stent placement have been developed and put to clinical use. Such a drug-eluting stent has a structure in which a surface thereof is coated with a hydrogel polymer such as polyacrylic acid (see, for example, WO-A1-92/11896).
The stent as described in the above Patent Document WO-A1-92/11896 has its surface entirely coated with the hydrogel polymer. When such a stent is put in a physiological environment (particularly, in a blood vessel), expansion of the stent and swelling of the hydrogel polymer occur simultaneously, and the hydrogel polymer particles come into contact with each other, resulting in an excessive increase in volume. Therefore, strains are generated between the stent and the hydrogel polymer, whereby the hydrogel polymer is caused to peel off the stent and flow out into the blood vessel.